A number of mechanisms have been proposed to explain the depletion of help- er CD4+ cells in HIV-infected individuals. Gougeon and Montagnier suggested that loss of CD4+ cells in HIV-infected patients is associated with apoptosis (programmed cell death). We have focused on the role of oxidative stress in apoptosis and observed that the ability of a drug, caffeic acid phenethyl ester (CAPE), to induce apoptosis is related to the redox state of the cell. CAPE has growth inhibitory effects on normal cells and differential toxicity to certain neoplastic cells. The mechanism of CAPE- induced differential cytoxicity was investigated by using normal cloned rat embryo fibroblasts (CREF) cells, a clone of type 5 adenovirus- transformed CREF cells, Wt3A, and a human colon carcinoma cell line, RKO. Treatment with CAPE resulted in morphological changes and extensive DNA degradation in Wt2A cells, typical of apoptosis. Apoptosis of Wt3A cells could be induced also by depletion of serum from growth medium, confluent cell culture, and treatment with cyhcloheximide (a protein synthesis inhibitor), and diamide (a thiol oxidation agent). CREF cells were resistant to all the above treatments. However, depletion of glutathione rendered CREF cells as susceptible at Wt3A cells to CAPE- induced cytotoxicity. Pretreatment of Wt3A cells with either glutathione of N-acetylcystein- protected Wt3A cells from CAPE-induced apoptosis. CAPE also induced apoptosis in RKO cells. The role of p53 in CAPE-induced apoptosis was investigated by expressin human papiloma virus E6 oncogene in RKO cells. Our results suggest that loss of normal growth control mechanisms in Wt3A and RKO cells may be responsible for induction of apoptosis under various conditions. CAPE may function to induce cell death through redox-regulated mediators in the cells. We will examine the effect of CAPE on tat expressing cells in the future as a chemotherapeutic agent. Preliminary studies indicate that antioxidants can block tat-induced apoptosis.